1. Field of the Invention
The present invention relates to a wavefront measuring apparatus for measuring a wavefront of a light beam by means of optical interferometry, particularly, to a wavefront measuring apparatus for optical pickup appropriate to measure a wavefront of a light beam mixed with a sub beam for a tracking adjustment, which is output from the optical pickup apparatus.
2. Description of Related Art
As wavefront measuring apparatuses that measure a light beam by means of optical interferometry, optical system arrangement based on Mach-Zehnder interferometer has been generally used. However, recently, wavefront measuring apparatuses using optical system arrangement based on Fizeau interferometer or optical system arrangement based on Michelson interferometer have been put to practical use (see JP-A-2005-345441 and JP-A-2006-343121).
Fizeau type and Michelson type wavefront measuring apparatus is characterized in that a reflection-type wavefront shaping unit is used as means for making a reference beam from a light beam to be measured. The reflection-type wavefront shaping unit has an optical element (hereinafter, it is referred to as a “reflecting/diffracting section” or “reflecting and diffracting section”) that shapes a wavefront of the light beam to be measured, by reflecting a part of a beam for reference beam generation, which is split from the light beam, in a direction opposite to an incident direction. In such a reflecting/diffracting section which is also called a reflection type pinhole, it has been known to form a micro reflection area on a glass substrate, to form a micro reflection area on a front end of a needle-shaped member (see JP-A-2000-097612), to dispose a reflective surface just close to the rear side of a normal pinhole (see JP-A-58-60590), or the like.
The wavefront measuring apparatuses as described above are used for measuring a wavefront of a light beam output from an optical pickup apparatus for reading out signals of an optical disk such as CD or DVD. In the optical pickup apparatus, three beam method of: splitting a light beam emitted from a light source into a main beam for reading data pit column and two sub beams for a tracking adjustment; and outputting the beams is generally used.
Since the light beam output from the optical pickup apparatus has the main beam and the sub beam mixed with each other, in a case where an interference fringe image thereof is taken by inputting the light beam into the wavefront measuring apparatus, an undesirable interference fringe component caused by the sub beam is superposed on the taken interference fringe image. Hence, even when the interference fringe image is analyzed, it is difficult to obtain favorable result of wavefront measurement for the main beam.
Accordingly, it can be also considered that the sub beam is blocked by using a pinhole plate and only the main beam is selectively input to the wavefront measuring apparatus. However, it is difficult to precisely dispose the pinhole plate on a position where the sub beam can be blocked and only the main beam can pass, and it takes much time. In particular, in a case of an optical pickup apparatus configured to be able to output a light beam having a plurality of mutually different wavelengths corresponding to standards of optical disks, it is necessary to change a diameter of the pinhole in accordance with a wavelength of the light beam to be measured. Thus, a lot of time is needed for the measurement.